1. Field of the Invention
The present invention relates to a surface emitting semiconductor and a method for manufacturing the surface emitting semiconductor. In particular, the present invention relates to a surface emitting semiconductor used as a light source for optical information processing and optical communication and as a light source for a data storage device using light and to a method for manufacturing the surface emitting semiconductor.
2. Description of the Related Art
In recent years, the need for a surface emitting semiconductor laser in which a light source is easily arranged in a two-dimensional array (Vertical-Cavity Surface Emitting Laser Diode which is hereinafter referred to as VCSEL) is increasing in the technical field of optical communication and optical recording and the like.
The surface emitting laser has advantages of having a low threshold current and lower power consumption, easily producing a circular optical spot, and being excellent in productivity because it can be evaluated in the wafer state. Conversely, it has been known that the volume of an active region is small, and thus the threshold current becomes lower, increases device resistance to a range from several tens Ω to several hundreds Ω and makes it difficult to increase the optical output power of the device to a range from several mW to several tens mW.
Recently, a short-haul optical communication (from several meters to several hundred meters) using a multimode type optical fiber which is manufactured at low cost and is typically a plastic optical fiber (POF) has received much attention. A combination of a single-mode optical fiber and a comparatively long wavelength laser having a wavelength of 1.3 μm or 1.55 μm is used for a long-haul optical communication, but these are expensive and not suitable as consumer goods since the cost is too high. On the other hand, the light source device itself used for the multimode type optical fiber needs to be inexpensive, there should be no need for a special optical system and a driving system and the multimode optical fiber must be small in size and weight. For this reason, the surface emitting laser having these features is thought to be a promising device.
Among the typical surface emitting lasers now available in the market is a proton injection type VCSEL. In the proton injection type VCSEL, a small difference in refractive index is produced by a thermal lensing effect caused by heat between a region through which a current passes and the peripheral region thereof to produce a state of weak light confinement. According to this principle, the diameter of a non-proton injection region (current path) is made ten to several tens μm to produce a laser oscillation. However, it is also known that because a luminous efficiency is reduced by the weak light confinement and heat generation is large, the threshold current is high and a response performance is not good in the state where a bias voltage is not applied thereto.
The proton injection type VCSEL is referred to as a “gain wave guide structure” when classified based on structure. Meanwhile, the type in which a refractive index distribution for light confinement is specifically formed, is a selective oxidation type VCSEL which is classified as a “refractive index wave guide structure”. In this type of VCSEL, a refractive index waveguide is formed by selectively oxidizing a part of a semiconductor multilayer reflection film near an active region to produce a strong light confinement effect, so this type of VCSEL has a low threshold current and a high response performance.
However, even in the selective oxidation type VCSEL showing a good performance, if the diameter of a light emitting region (nearly corresponding to the diameter of a non-selective oxidation region) is enlarged for the purpose of increasing an output power, the VCSEL is also allowed to produce oscillations of various orders, that is, produces a so-called multimode oscillation. In the multimode oscillation, a spectral line width is made wide and the optical fiber has the mode dispersion characteristics, so the attenuation of signal in the fiber is increased, or a mode state is made unstable and thus the main order of mode of the oscillation is easily varied by a change in the amount of current injected and a change in the environmental temperature. A dynamic change in the mode order is not preferable because it changes a coupling efficiency with the fiber.
To avoid this problem, there is a method of controlling an oscillation transverse mode so as to oscillate only in a fundamental mode of the lowest order (0 order) by making the diameter of the light emitting region smaller.
However, the diameter of the light emitting region of the VCSEL needs to be reduced, typically, to 4 μm or less, which is smaller than that of the above-described proton injection type VCSEL, and thus these VCSEL has a defect of having a high element resistance and being unable to produce high output power. Making the transverse mode stable is an important requirement for preventing the signal from being attenuated when the VCSEL is optically coupled to the optical fiber. In addition, it is necessary to improve electric optical characteristics.
Among ideas for simultaneously realizing opposing goals of making the transverse mode stable, and reducing resistance and increasing output power in the selective oxidation type VCSEL having excellent luminous efficiency and high response performance, is a VCSEL having a structure disclosed in IEEE Photonics Technology Letters, Vol. 11, No. 12, page 1536-1538 (see FIG. 13). In this example, the diameter of the light emitting region is as large as 20 μm but the inside of an electrode aperture emitting laser light is etched away to a depth of 40 nm except for a region of a radius of 7.75 μm from the center of the aperture.
The report discloses the following: Since the diameter of the light emitting region is as large as 20 μm, in the case where there is no surface processing, the order of oscillation mode is varies in accordance with the amount of injection current and thus a far-field image is observed to vary; in contrast, a surface emitting semiconductor laser with a hole produces a fundamental mode up to an optical output of 0.7 mW but when current exceeding that level is injected, the mode splits to gradually widen the far-field image.
The purpose of the VCSEL described above is to improve the optical output power in the fundamental mode. However, the maximum optical output power of the surface emitting semiconductor laser with a hole is 10.4 mW, whereas the output power in the fundamental mode is only 0.7 mW. Taking into account that the maximum output power in the case where there is no surface processing is 17.9 mW, the report described above clearly shows that it is very difficult to make the transverse mode stable and to produce a large optical output power at the same time.
In this respect, various other VCSEL structures for controlling the mode have been proposed. For example, U.S. Pat. No. 5,940,422, filed Jun. 28, 1996, assigned to Honeywell Inc. discloses a VCSEL in which a mode control is performed by forming two regions of different film thicknesses. In this invention, only a region on which an additional film is deposited becomes a light emitting region. It is thought that the purpose of the invention is to artificially determine the position of a light emitting spot and not to determine the position by taking into consideration the specific oscillation to be produced in the VCSEL (for example, the oscillation mode of producing five light emitting spots, described as one preferred embodiment, does not exist in the natural world).
Further, U.S. Pat. No. 5,963,576, filed Aug. 4, 1997, assigned to Motorola Inc. discloses a VCSEL having an annular waveguide. It is thought that the purpose of the invention is to produce a mode in which light emitting spots are arranged regularly in an annular region so as to produce a super resolution spot and not necessarily to deliberately produce a specific oscillation mode of a determined order.
IEEE Photonics Technology Letters, Vol. 9, No. 9, published in September 1997, page 1193-1195, authored by University of Bristol and Hewlett Packard Labs. discloses a VCSEL having a configuration in which a circular cavity is formed on the top surface of a post by etching to locally vary a mirror reflectivity. The report discloses that the spectral line width of this device is reduced to a half of that of a device with no cavity to produce an effect of suppressing the mode. However, as the amount of current injected increases, an oscillation spectrum is observed to vary. This clearly shows that a specific oscillation mode is not always dominant, in other words, that the mode is not stable.
Further, Electronics Letters, Vol. 34, No. 7, published in April 1998, page 681-682, by Motorola Inc. proposes a VCSEL having a configuration in which a circular cavity is formed on the top surface of a post by etching and in which an annular light emitting region is formed on the outer peripheral portion of the cavity. It is clear from a near-field pattern that a very high order (larger than 30th order) mode is produced and at the same time that there are large variations in the intensity of light emitting spot. This shows that it is difficult to inject a uniform current into the annular region of an inside diameter as large as 30 μm. Therefore, there is plenty of room for improvement of the VCSEL in order to obtain a stable high order mode oscillation for practical application.
As described above, as to the VCSEL expected as a light source for a multimode type optical fiber, the state of art in the VCSEL technology can not provide a device that satisfies a requirement of stabilizing a transverse mode and has high output power, low resistance, high efficiency and high speed response.